Process Control For Ozonation Systems: A Novel Real-Time Approach

Abstract

For real-time control of ozonation processes in water works, a sequencing batch reactor was constructed to measure the ozone decay rate constant (k_O3) in short time intervals of about 15 min. The batch reactor is filled during the production process, immediately after dissolving ozone in water by a static mixer. On the basis of k_O3 and the initial ozone concentration ([O₃]₀), and the experimentally determined ratio of the concentrations of ^•OH radicals to ozone (R_ct), the degradation of micropollutants in ozone reactors (modeled as Continuously Stirred Tank Reactors - CSTRs) were calculated for compounds with known reaction rate constants with ozone and ^•OH radicals. Calculated degradation of atrazine, iopromide, benzotriazole and acesulfame are in good agreement with measured data. For acesulfame the following rate constants were determined in this study at 20 ^oC: reaction rate constant with ozone = 88 M⁻¹s⁻¹, reaction rate constant with ^•OH radical = 4.55 × 10⁹ M⁻¹s⁻¹. For the ozone reaction an activation energy of 35 kJ/mol was determined. Similarly to micropollutants, the relative inactivation of microorganisms (N/N₀) can be calculated based on the inactivation rate constant for ozone and if applicable the lag phase. The pI-value (= −logN/N₀) was introduced and implemented in the process management system to calculate online the log inactivation of reference microorganisms such as B. subtilis spores. The system was tested for variation of pH (6.5–8.5), DOC (1.2–4.2 mg/L) flowrate 3.2–12 m³/h and temperature (5.7–9 ^oC). Furthermore, a given pI-value, e.g. 1 for a 1-log inactivation of B. subtilis spores, can be set as control parameter in the process management system. The ozone gas flow is then adjusted until the set pI-value is reached. The process control concept was validated with B. subtilis spores. Generally, a good agreement was found between calculated and measured inactivation data. It was also demonstrated, that a constant ozone residual may lead to insufficient disinfection or overdosing of ozone. The new process control concept for ozonations based on onsite measurement of the ozone decay rate constant and the pI-value allows to assess disinfection and degradation processes quantitatively in real-time.

Keywords:

• Ozone
• Second-Order Rate Constants
• Acesulfame
• Ozone Decay Rate Constant
• Degradation of Micropollutants
• Disinfection
• Bacillus Subtilis Spores
• Reactor Hydraulics
• Process Management System
• Process Control

ACKNOWLEDGMENTS

Sarah Pati and Marcel Leemann provided the micropollutants degradation experiments, WABAG AG, Switzerland constructed the ozone reactor and the final version of the batch reactor and Chestonag Automation AG, Switzerland implemented the pI calculation and the pI control system in the process management system.